Question

2.When concentrated H2SO4 (18M) is added to a saturated solution of sodium sulfate, a white precipitate is formed. Write a net-ionic equation for the equilibrium in saturated sodium sulfate, and explain the change in terms of Le Chatelier’s principle.

3.A solution contains the blood-red hexathiocyanatoferrate(III) complex ion, Fe(SCN)63-, in equilibrium with the iron(III) ion, Fe3+, and six thiocyanate ions, SCN-. If the solution is diluted with water, will the color deepen, fade, or stay the same? Why? In addition to predicting whether the solution will deepen, fade, or stay the same, give TWO reasons for that prediction. One of those reasons will involve Le Chatelier’s Principle, and one will not. (At least, that will be the case if you make the right prediction!)

Answer 1

2. When we add highly concentrated H2SO4 to Na2SO4 solution, the concentration of the common ion(SO₄2-) increases. On continuous adding the concentration of the sulphate ion(SO₄2-) becomes so high that it becomes over-saturated. Now according to Lechatelier's principle, in order to maintian the equilibrium sulphate ion(SO₄2-) concentration must decrease. Hence sulphate ion(SO₄2-) is precipitated as white colored Na2SO4(s). The net ionic equation is

2Na+(aq) + SO₄2-(aq) ------- > Na2SO4(s)

(3) The dissociation reaction in equilibrium is

Fe[(SCN)₆]^3------- > Fe³⁺(aq) + 6SCN^-(aq)

When the solution is diluted, the concentration of Fe³⁺(aq) and SCN^-(aq) decreases. Now according to Lechatelier's principle in order to maintain the equilibrium constant, the concentration of Fe³⁺(aq) and SCN^-(aq) must increase. Hence more and more Fe[(SCN)₆]^3- are dissociated. Hence the blood red color will fad away.